The manufacture of very uniform particles of different substances is of commercial significance. In the field of polymer chemistry the creation of uniform spheres of polymers is useful in a number of applications such as calibration of scientific equipment (e.g. Electron Microscopes) and in, for example, the manufacture of uniform size beads of ion exchange resins.
There are a number of patents, which teach the formation of uniform droplets of one or more monomers in a continuous medium and these uniform size monomer droplets are further subjected to conventional batch emulsion or suspension polymerization. The polymerization includes some sort of mechanical agitation. Typically, mechanical agitators generate a shear gradient low at the center of the agitator and higher at the peripheral edge of the agitator. As a result, the mono-sized droplets of monomer are subject to a non-uniform shear field under conventional mechanical agitation and this tends to broaden the particle size distribution towards a bell curve type distribution (e.g. Gaussian distribution).
U.S. Pat. No. 4,623,706 issued Nov. 18, 1986 to Timm et al. assigned to the Dow Chemical Company discloses a process in which monomers are atomized into a gas phase containing column. Part of the gas is a cationic polymerization initiator. In one embodiment the dispersed monomer droplets travel through the gas phase column and are substantially polymerized before leaving the column (Col. 6 lines 16 and 44 and 45). In another embodiment the droplets are sprayed onto a wall wetted with a continuous phase, which flows into a second reactor where the droplets are polymerized using conventional emulsion or suspension polymerization techniques. The specification discloses the use of a mechanical agitator (41) in the reactor. The reference teaches away from the subject mater of the present invention.
U.S. Pat. No. 4,444,961 issued Apr. 24, 1984 to Timm assigned to The Dow Chemical Company teaches a process for producing uniform droplets of one or more monomers in a continuous liquid phase. The monomer droplets are injected into a column containing the continuous phase. In the column the continuous phase may flow co-currently or counter currently to the movement (e.g. floatation or sedimentation) of the droplets or be static (Col. 7 lines 19–33). However the conditions in the column are such that no or very low polymerization occurs (Col. 7 lines 34 and 35 and Col. 13 lines 23 to 30) and the droplets are subsequently transferred to a conventional emulsion or suspension reactor. The turbulence level in the reactor is maintained low to prevent agglomeration, however the reactor clearly contains some type of agitation (Col. 14 lines 16 to 29). Again the reference teaches away from the subject matter of the present invention.
U.S. Pat. No. 4,666,673 issued May 19, 1987 to Timm assigned to The Dow Chemical Company teaches a process in which uniform droplets of one or more monomers are injected into a continuous phase. The continuous phase is in a collection zone and the resulting droplets are fed to a conventional emulsion or suspension reactor. Within the collection zone the flow of the continuous medium is described in a similar manner as in U.S. Pat. No. 4,444,961 (Col. 5 lines 27 to 65). Again the resulting droplets are further fed to a conventional reactor, which is described in a similar manner as in U.S. Pat. No. 4,444,961 (Col. 8 lines 46 to 60). The disclosure teaches away from the present invention.
U.S. Pat. No. 4,427,794 issued Jan. 24, 1984 to Lange et al. assigned to Bayer Aktiengesellschaft teaches a process for producing uniform droplets of monomers which are encased within a shell within a continuous liquid phase. The monomer droplets are then transferred to a conventional emulsion or suspension reactor having mechanical agitation. The patent teaches away from the subject mater of the present invention.
U.S. Pat. No. 5,061,741 issued Oct. 29, 1991 to Miyata et al. assigned to Mitsubishi Kasei Corporation teaches a process for preparing uniform droplets of one or more monomers in a continuous phase. The resulting droplets are fed to a conventional emulsion or suspension reactor. The disclosure recognizes the problems with particle break up or agglomeration and recognizes the need to use mild stirring, but teaches mechanical agitation. The patent teaches away from the subject matter of the present invention.
U.S. Pat. No. 6,610,798 issued Aug. 26, 2003 to the same inventors as the currently submitted application, assigned to NOVA Chemicals Inc., discloses a process for producing mono-sized monomer droplets which are polymerized in a low shear zone in a reactor created without mechanical agitation, by injecting a gas inert to the reactor contents into the continuous phase. The patent does not teach re-circulating a portion of the continuous phase to create such a zone.
The applicants have been unable to locate any art, which discloses the subject matter of the present invention (i.e. the re-circulation of all or a portion of the continuous liquid phase to create a low shear, low turbulence, (preferably laminar), flow pattern within a continuous phase contained in a vessel). If the continuous phase optionally further comprises a dispersed phase, the present invention provides the mechanism to create, with no mechanical agitation, a laminar low shear flow pattern to suspend and to distribute, preferably relatively uniformly, the dispersed phase within a volume or a portion of the continuous phase, with minimum interaction between particles during subsequent processing. The application of such a flow pattern can be useful in any process where the substrate (liquid or solid) is dispersed as a suspension or an emulsion or as a particle cloud in an immiscible continuous phase and the dispersed phase may be further batch processed or undergo a chemical reaction (e.g. polymerization), with the requirement to substantially (e.g. 90%, most preferably 95%) preserve the initial size distribution.
In conventional processes, a phase to be processed, dispersed in a continuous liquid phase is usually subjected to some form of mechanical agitation. If the initial particle size distribution of the dispersed phase, which may have been obtained by any means of disintegration including pressure atomization, extrusion, mechanical agitation, jet cutting, etc., is to be maintained during further processing or chemical reaction, the flow pattern of the continuous phase in a rector needs to have low enough shear and low turbulence level so as not to cause particles to agglomerate or not to cause further particle break up. With conventional mechanical agitation it is difficult to generate such a low turbulence, uniform shear field as the velocity gradient along an agitator blade is a function of the liquid properties, the speed of rotation and the distance from the agitator shaft. Accordingly, the shear in the reactor vessel is highly non-uniform and generally the level of turbulence is high and this tends to change the initial particle size distribution into a new, usually a normal, distribution of particle sizes.
The present invention seeks to provide a method of generating a low shear, low turbulence, in some applications preferably laminar, zone within a continuous liquid phase in a vessel, with no mechanical agitation in the reactor. If dispersed particles are comprised in the continuous liquid phase, they flow within this zone with minimum mutual interaction to minimize agglomeration and breakups, remaining submerged and thoroughly (uniformly) distributed within the continuous liquid volume during subsequent processing. In such environment, the initial particle size distribution can be largely preserved.
The method can be applied in processes where density ratios between dispersed and continuous phases are higher or lower than 1 within the range of ±20%.